It is well known that the impulse response of a graded-index, multimode fiber of given size and composition is a function of the index profile. In particular, it has been shown that for each wavelength there is an optimum power law distribution defined by an optimum power law coefficient .alpha. close to 2 for which the rms pulse width is a minimum. (See, for example, D. Gloge and E. A. J. Marcatili, "Multimode Theory of Graded-Core Fibers," Bell System Technical Journal, Vol. 52, No. 9, November 1973, pp. 1563-1578; and U.S. Pat. No. 3,823,997.) Small departures from this optimum .alpha.-value cause the rms pulse width to increase dramatically. Thus, it has been considered very important to design graded-index, multimode fibers such that their refractive index profiles conform very closely to the optimum .alpha.-value.
More recently, however, there has been experimental evidence that fibers with non-optimum .alpha.-values can be cascaded to yield a narrower impulse response than is obtained for each of the fibers individually inasmuch as the fibers tend to complement each other such that the slower modes in one become the faster modes in the other and vice versa. (See, M. Eve, "Multipath Time Dispersion Theory of an Optical Network", Optical and Quantum Electronics, Vol. 10, No. 1, January 1978, pp. 41-51; M. Eve, A. Hartog, R. Kashyap and D. N. Payne, "Wavelength Dependence of Light Propagation in Long Fibre Links", Fourth European Conference on Optical Communication 1978, Symposium Digest, pp. 58-61; and M. Eve, P. C. Hensel, A. M. Hill, J. E. Midwinter, B. P. Nelson and J. R. Stern, "Transmission Performance of Three Graded Index Fibre Cables Installed in Operational Ducts", Third European Conference on Optical Communication 1977, Symposium Digest, pp. 53-55.)
At present, fiber preforms are designed for the optimum .alpha.-value. However, controlling the fabrication process to achieve this is a difficult and, correspondingly, expensive process. Even with care, the index profiles in the resulting preforms and the fibers that are drawn therefore are rarely, if ever, optimum. To attempt to compensate for this by cascading different fibers is, at best, a haphazard solution to the problem.
A second problem affecting the impulse response of optical fibers resides in the manner in which fiber preforms are made. Typically, preforms are made by the successive deposition of layers of materials having slightly different refractive indices. Thus, the actual index profile is not a simple power law function but, rather, a power law upon which a small perturbation is superimposed. However, it is also well known that irregular departures of the refractive index profile from the ideal distribution also cause a broadening of the impulse response. (See, for example, R. Olshansky, "Pulse Broadening Caused By Deviations From The Optimal Index Profile", Applied Optics, Vol. 15, No. 3, March 1976, pp. 782-788; and D. Marcuse, "Calculations of Bandwidth From Index Profiles Of Optical Fibers, Part I: Theory", to be published in Applied Optics.) Thus, there are two factors which affect the signal bandwidth of an optical fiber. The first is the inability to realize the optimum index profile for a given operating frequency. The second, inherent in the preform fabrication process, relates to the perturbations on the index profile. Accordingly, it is the broad object of the present invention to minimize these deleterious effects upon fiber performance.